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Saturday, December 1, 2001

How Vulnerable is GPS?

A study conducted by the John A. Volpe National Transportation Systems Center outlines the problems that can occur from both deliberate and unintentional interference with the GPS signal. Should Loran-C be a backup?

Charlotte Adams

On Sept. 10, the U.S. Department of Transportation (DoT) released a report authored by its John A. Volpe National Transportation Systems Center, on the vulnerability of the U.S. transportation infrastructure in relying on the Global Positioning System (GPS). It was slow in coming. The vulnerability assessment saw the light of day more than three years after a presidential directive in May 1998 had assigned the task to DoT. Government security concerns delayed publication for many months, but public demand, coupled with secretarial-level interest, finally secured the study’s release and gave it high visibility.

The Federal Aviation Administration (FAA) and other department entities are to review the report, "consider the adequacy of backup systems for each area of operation in which GPS is being used for critical transportation applications," and report back within 60 days, says DoT. Since the 2001 Federal Radionavigation Plan (FRP) was well advanced before the Sept. 11 tragedy in New York City and at the Pentagon, the results of current reviews will be better reflected in the FRP for 2003. (However, the 2001 edition of the biennial plan will synopsize the Volpe Center’s recommendations and reflect DoT’s request for reviews and responses.) Because of the time lapse between the study’s completion and publication and the evolving vulnerability picture, much of the information in the report is now somewhat stale. The idea is being raised of regular updates, similar to those of the FRP.

Not Alarming, But…

While not alarmist, the Volpe study torpedoes the conclusion of an earlier, FAA-funded (and rapidly disseminated) Johns Hopkins University study which supported GPS for sole means navigation. "The implications of sudden loss of GPS over major population areas, possible long-term and widespread GPS outages, numerous reports of large undetected position errors due to jamming, and the potential for a counterfeit signal to induce position errors are just too serious, both for safety and continuity of operations" to endorse GPS as the only navigation system for operations in the National Airspace System (NAS), the Volpe experts say.

The Volpe report calls, among other things, for thorough cost/benefit analyses:

  • Defining the risks to critical applications,

  • Deciding the levels of acceptable risk,

  • Determining the costs of lowering the risks to acceptable levels, and

  • Resolving the means of funding those costs.

It discusses various unintentional and intentional disruptions to service and outlines mitigations. The "bottom line [is that] … awareness and planning can mitigate the worst vulnerabilities," said Jim Carroll, a Volpe senior project engineer and a co-author of the study.

The report addresses the vulnerabilities of all the transportation modes and notes the use of GPS as a timing reference for the national power grids and telecommunications systems. The study devotes particular attention to aviation, which will become more reliant on GPS, as augmentation systems are phased in and the number of ground-based navaids is reduced. It notes, for example, GPS dependencies in future communications (Nexcom) and surveillance systems (Automatic Dependent Surveillance-Broadcast).

The Volpe report identifies vulnerabilities in the GPS system: it employs extremely low-power signals, only one of which is available for civil aviation use. The ongoing GPS modernization plan will add another, stronger civil signal in the protected band and will make unintentional interference less likely. But full operational capability for the new signal is not expected until 2014.

The Volpe study discusses unintentional GPS disruption, such as ionospheric interference and radio frequency (RF) interference from the likes of broadcast television, VHF transmitters, personal electronic devices, Mobile Satellite Service (MSS) communications systems, and ultra-wideband (UWB) radar and com systems. It also discusses intentional disruptions, from "shutdown" through attacks on the satellites or the ground control segment to jamming, spoofing (fooling receivers with false signals) and meaconing (the reception, delay and rebroadcast of signals to confuse a nav system).

Jamming is the most common mode of intentional disruption. Russian handheld 4-watt jammers are said to disrupt the signal over an area 100 nautical miles in radius. Jamming devices are available and can be easily built, Carroll said. One-watt jammers, the size of a Coke can, can easily be moved around and deployed. "There is a fairly large GPS disruption industry," he said.

The report recommends continuing the GPS modernization program and spectrum protection efforts, as well as assessing military anti-jam technology for civil use. It also advocates coordinating with the U.S. Defense Department on the availability of anti-spoofing technologies and threat information, an ambitious agenda.

Re-evaluating Ground Navaids

Concerning aviation, the Volpe study recommends a "comprehensive analysis of GPS backup navigation and precise timing options, including VOR/DME, instrument landing systems, Loran-C, inertial navigation systems, and operating procedures." FAA is evaluating the current navaid phase-down schedule, commented Kelly Markin, program manager for navigation with FAA consultant Mitre Corp. "The originally planned decommissioning is being delayed," he said, "and will occur some number of years later than 2008," the date mentioned in the current FRP.

Markin said that, although there has been some discussion in the past about reducing en-route primary radar capabilities, "they are back in the picture because of the 9/11 event and the use of the radars as part of the Homeland Defense capability, a new office designated by President Bush."

Former FAA Administrator Langhorne Bond contends that the eventual, residual aviation backup network should be able to carry the same capacity as the GPS-anchored system. "If there is a long-term outage, we must keep the air traffic control system going with the same level of sophistication and service as before," he said. "It’s not enough just to get the airplanes on the ground." Nevertheless he agreed that much of the ground-based network could be removed when GPS satellite navigation is in place.

Is Loran the Answer?

The John A. Volpe National Transportation Systems Center report on GPS vulnerability recommends the continuation of the FAA/U.S. Coast Guard Loran-C modernization program. With its higher-power signal and precise timing features, Loran may be an appropriate backup.

FAA has demonstrated the use of Loran to broadcast Wide Area Augmentation System (WAAS) differential correction messages. The agency is evaluating whether Loran can meet the availability, accuracy, integrity and continuity requirements to support lateral navigation (LNAV) during approach, including missed approach guidance.

Said Kelly Markin, program manager for navigation with Mitre Corp.: "Loran-C could be made to work for nonprecision approach. It has been considered as a possible supplement to the BBN [basic backup network]…for low-altitude, en-route coverage, but the added cost for users to equip has been an issue."

The U.S. government is continuing Loran in the short term, pending a decision on its long-term usefulness. Thanks to congressional mandates, the system is being recapitalized, but "its new life will be short-lived" if none of the transportation modes selects it and "activates industry participation," said Jim Carroll, a Volpe senior project engineer and study co-author. FAA designates Loran as a supplemental nav system.

The Volpe report talks about the impact of GPS outages during en-route and terminal navigation segments of flight on the air traffic control (ATC) system. "Serious" outages (minutes or hours) will require an alternate procedure or possibly a backup system, the report says. "Controller vectoring could probably maintain safety, but this assumption needs to be thoroughly validated." The report cites FAA plans to simulate GPS outages (see article, page 31).

No Shortage of Viewpoints

At the public meeting held by the U.S. Department of Transportation to gather user input regarding the John A. Volpe National Transportation Systems Center study of GPS vulnerability, and in separate interviews, some strong opinions were aired. Here are several examples:

John Beukers
International Loran Association

"The nail is finally in the coffin of … GPS as sole means. It’s not safe. It’s almost criminal."

Langhorne Bond
Former FAA Administrator

Bond described a potentially dangerous reliance on a single system: "GPS positioning and/or timing [has] crept into all three elements of the NAS [National Airspace System]–communications, navigation and surveillance. Now we have a common failure mode that could bring down the entire ATC [air traffic control] system." Bond stressed the potential of Loran to provide a backup, jam-resistant timing source and offer nonprecision approach capability.

Patrick Reines
Regional Manager-Airport Systems

"There are a lot of ways to mitigate various threats … Somebody has to decide what is the risk, what are the consequences, what is the acceptable level that we go and design against, build against. [But] what’s the process to keep this from being dragged out through years and years of debate? … The traditional methodologies [to reach] consensus … have really slowed down getting on with putting equipment out that can help, from our perspective, users of the NAS system." (Honeywell is a participant in the Local Area Augmentation System [LAAS] program.)

Bob Jackson

Business Development Manager-FAA Programs
Raytheon Co.

"The report did a good job on the vulnerabilities of GPS. But there was no discussion of the vulnerabilities of other systems. It’s a fact of physics that any [electromagnetic] system is vulnerable. It’s a question of the balance of the report."

Jackson said: "Even when Loran-C is functioning properly, it has a tendency to jam itself," through a phenomenon known as cross-rate interference. "You can be in a position in which you’re receiving signals from two chains and, depending on how the receiver processes those, they interfere with each other." (Raytheon is the developer the Wide Area Augmentation System [WAAS].)

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